While less susceptible to optimization problems than left and right,
interval_tree_iter_next also reads rb_parent(), so make sure that
stores and loads are atomic.

This goes further than technically required, changing all loads to
be atomic, rather than simply the ones in the iteration side.  But
it doesn't really affect the code generation on the rebalance side
and is cleaner to handle everything the same.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>